A dealer should NEVER open a private sale transfer gun unless they want to be solely responsible for any reduction in condition from NIB. This is seriously in poor taste, and if buyer and seller cannot resolve the issue of condition, the dealer is the only other person who handles the gun.

A firearm should ALWAYS be viewed before the transfer. The dealer may hold your money while you look at it, but the transfer is a matter of permanent record. What if this were a new Glock, but you inspected it after the transfer and found it damaged? Most dealers won't transfer it back, especially if they aren't selling it.